299 research outputs found
Energy Saving Potential in Existing Volumetric Rotary Compressors
The issues of energy and Carbon saving in energy intensive sectors, along with that of energy generation from renewable sources, have been recently receiving a growing awareness, as they are perceived as the most effective ways to deal with global sustainability commitments. The Compressed Air Sector (CAS) accounts for a 10% worldwide electricity consumption, and thus is being re-thought as an area offering great opportunities for improvement. Considering that the compression is responsible for a 10-15% consumption, it is vital to pay attention to machines performances. An overview of present compressor technology is given and saving directions for Screw and Sliding Vanes machines are analysed: interesting source of information was the Compressed Air and Gas Institute (CAGI), whose data have been processed, in order to obtain consistency with fixed reference pressures, and organized as a function of main operating parameters. The overall efficiency has been split and all different sub-terms (adiabatic, volumetric, mechanical, electrical, organic) considered separately. This has allowed a term-by-term evaluation of the margin for improvement. The heat recovery from the oil into mechanical energy via an Organic Rankine Cycle (ORC), together with the thermodynamic improvement during compression phase opens the way to a step change concerning the specific energy consumption. The Authors acknowledge Ing. Enea Mattei S.p.A. and particularly its CEO, Dr. Giulio Contaldi, for continuous research funding and support. The work has been done also under the FP7 Project "Complete Vehicle Energy-Saving CONVENIENT" funded by the European Commission
Quasar Evolution Driven by Galaxy Encounters in Hierarchical Structures
We link the evolution of the galaxies in the hierarchical clustering scenario
with the changing accretion rates of cold gas onto the central massive black
holes that power the quasars. We base on galaxy interactions as main triggers
of accretion; the related scaling laws are taken up from Cavaliere & Vittorini
(2000), and grafted to a semi-analytic code for galaxy formation. As a result,
at high the protogalaxies grow rapidly by hierarchical merging; meanwhile,
much fresh gas is imported and also destabilized, so the holes are fueled at
their full Eddington rates. At lower the galactic dynamical events are
mostly encounters in hierarchically growing groups; now the refueling peters
out, as the residual gas is exhausted while the destabilizing encounters
dwindle. So, with no parameter tuning other than needed for stellar
observables, our model uniquely produces at a rise, and at a decline of the bright quasar population as steep as observed. In addition,
our results closely fit the observed luminosity functions of quasars, their
space density at different magnitudes from to , and
the local relation.Comment: 5 pages. Accepted for publication in ApJ Letter
Relic Black Holes In Galactic Nuclei
We discuss the link between the observations of distant quasars and those of
massive dark objects in the cores of many local galaxies. We show how the
formation of early black holes gives rise to the luminosity function of high z
quasars, while it imprints into their dark local relics a related shape of the
mass-dispersion correlation. We propose that in its lower section the
correlation slope will tell the (otherwise uncertain) strenght of the feedback
effect from the quasar radiation on the host galaxies.Comment: 4 pages, 3 figures, submitted to Ap
The Fall of the Quasar Population
We derive quantitative predictions of the optical and X-ray luminosity
functions (LF) for QSs in the redshift range . Based on BH paradigm, we
investigate how the accretion is controlled by the surrounding structures, as
these grow hierarchically. We argue that for efficient black hole
fueling is triggered by the encounters of a gas-rich host with its companions
in a group. The dispersion of the dynamical parameters in the encounters
produces a double power-law LF. Strong luminosity evolution (LE) is produced as
these encounters deplete the gas supply in the host; an additional, milder
density evolution obtains since the interactions become progressively rarer as
the groups grow richer but less dense. From the agreement with the optical and
the X-ray data, we conclude that the evolution of the bright quasars is
articulated in two ways. Earlier than the gas-rich protogalaxies grow by
merging, which also induces parallel growth of central holes accreting at
Eddington rates. In the later era of group assemblage the host encounters with
companions drive onto already existing holes further but meager accretion;
these consume the gas in the hosts, and cause supply-limited emissions which
are intermittent, go progressively sub-Eddington and peter out. Then other
fueling processes come to the foreground; we discuss the faint emissions,
especially noticeable in X-rays, which are expected when hosts in the field
cannibalize satellite galaxies with their meager gas contents.Comment: 12 pages Latex + 3 EPS figures, ApJ in press, we have corrected the
previous printing problems with the style \ca
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